1. Field of the Invention
The present invention relates to a conversion coating composition based on nitrogen and silicon compounds. More particularly, the present invention relates to a nitrogen and silicon compound-based conversion coating composition with excellent adhesion and acid resistance. Also, the present invention is concerned with a method for coating a printed circuit board with the conversion coating composition.
2. Description of the Prior Art
In the manufacture of multi-layer printed circuit boards (MLB), there is generally employed a black oxidizing process of inner circuits. Indispensable for the manufacture of an MLB, the black oxidizing process is conducted with the aim of protecting inner circuits and maximizing the bonding strength between inner layers and outer layers (bonding sheets, prepregs) when they are laminated.
Thus far, in the PCB industry, pretreatments, including cleaning with an alkali solution, rinsing, soft-etching, acid treating, and pre-dipping, have been taken in advance of a black oxidizing process of inner layers of an MLB to facilitate this subsequent process in which the inner circuits, consisting of copper, are oxidized black at a high temperature (70-80xc2x0 C.) by use of an oxidizing agent such as sodium chlorite (NaClO2) in an NaOH or KOH atmosphere.
Consisting mainly of cupric oxide (CuO), the black oxides formed as a result of such conventional blackening processes are highly vulnerable to acid to cause a serious pinkring problem. That is, when holes are formed during acid bath processes, subsequent to blackening and laminating processes, including an electroless plating process using soft-etching (H2SO4/H2O2), an acidic copper plating solution, and its pretreatments, the back oxides around the drilled holes are corroded by the acid used.
To circumvent this problem, the cupric oxide (CuO) layer is reduced to a cuprous oxide (Cu2O) layer, which is relatively resistant to acid, on the norm. This reduced layer, even though more resistant to acid than the CuO layer, cannot be an ultimate solution because it is dissolved upon the soft etching or in an inorganic acid such as sulfuric acid, hydrochloric acid and nitrogen acid. Further, cuprous oxide (Cu2O) is oxidized back to cupric oxide (CuO) at high temperatures.
In addition, the reduction of cupric oxide into cuprous oxide is achieved by use of a reducing agent such as dimethyl amine borane (DMAB), morpholine borane (MPB), formalin (HCHO), sodium borohydride (NaBH4), and potassium borohydride (KBH4). Of them DMAB and MPB are prevalently used because of their powerful reducing activity. However, they are very expensive and, indeed, encroach most of the expense required for the blackening process.
Besides, there are known several blackening processes that are conducted in acidic atmospheres. They suffer from the disadvantage of being very difficult to control because all are extremely sensitive to the chlorine ion concentration of the bath. Further, these blackening processes allow the black oxides to have only a peel or bond strength of less than 1 kg/cm2, so that they are poor in reliability.
Therefore, it is an object of the present invention to overcome the above problems encountered in prior arts, including alkali blackening processes and acidic blackening processes, and to provide a nitrogen and silicon compound-based conversion coating composition which is capable of forming on copper or copper alloy matrices a blackish brown oxide film superb in acid resistance and adhesion.
It is another object of the present invention to provide a conversion coating composition which allows the formation of blackish brown films on a PCB without requiring the pre- and post-treatment processes otherwise indispensable for conventional blackening processes.
It is a further object of the present invention to provide a conversion coating composition which provides excellent adhesion for the lamination of a multi-layer printed circuit board.
It is still another object of the present invention to provide a conversion coating method of a PCB by which a blackish brown oxide film superior in acid resistance and adhesion conversion can be formed on a copper or copper alloy printed circuit board.
It is still a further object of the present invention to provide a conversion coating method of a PCB, which requires no pre- and post-treatments otherwise indispensable for conventional methods and thus is economically remarkably favorable in terms of procedure and production cost.
In accordance with an aspect of the present invention, there is provided a conversion coating composition, comprising 0.1-30% by volume of sulfuric acid; 0.1-1 5% by volume of hydrogen peroxide; 0.01-10% by weight of an organic acid; 0.1-30% by weight of a nitrogen compound; 0.01-10% by weight of a silica and/or organic titanate; and at least one additive selected from a film forming auxiliary, an etching rate controller, a reaction promoter and a stabilizer, with a proviso that said film forming auxiliary, said etching rate controller and said reaction promoter are used at amounts of 0.1-20% by weight, 0.1-30% by weight, and 0.1-20% by weight, respectively, based on the weight of the conversion coating composition and said stabilizer is used at an amount of 0.1-20 g/l based on the volume of the conversion coating composition.
In accordance with another embodiment of the present invention, there is provided a method for forming a conversion coating film on a printed circuit board, comprising the steps of: cleaning and rinsing a copper or a copper alloy printed circuit board; applying to the printed circuit board a conversion coating composition comprising: 0.1-30% by volume of sulfuric acid; 0.1-15% by volume of hydrogen peroxide; 0.01-10% by weight of an organic acid; 0.1-30% by weight of a nitrogen compound; 0.01-10% by weight of a silica and/or organic titanate; and at least one additive selected from a film forming auxiliary, an etching rate controller, a reaction promoter and a stabilizer, with a proviso that said film forming auxiliary, said etching rate controller and said reaction promoter are used at amounts of 0.1-20% by weight, 0.1-30% by weight, and 0.1-20% by weight, respectively, based on the weight of the conversion coating composition and said stabilizer is used at an amount of 0.1-20 g/l based on the volume of the conversion coating composition to form a conversion coating film; and cleaning and drying the printed circuit board.